1. Field of the Invention
The present invention relates to chemical oxygen iodine laser's (COIL) and more particularly to a coil gain generator which provides enhanced efficiency, flow capacity and pressure recovery.
2. Description of the Related Art
COIL (Chemical Oxygen Iodine Laser) devices produce laser optical energy from energy stored in a form of oxygen known as singlet delta oxygen, (O.sub.2 (.sup.1 .DELTA.)). The O.sub.2 (.sup.1 .DELTA.) is typically produced in a gas liquid contact reactor (see e.g., U.S. Pat. No. 5,658,535, U.S. Pat. No. 5,624,654 and U.S. Pat. No. 5,516,502) from gaseous chlorine (Cl.sub.2) and liquid basic hydrogen peroxide (BHP). The BHP is a mixture of hydrogen peroxide and a strong base such as KOH. The gaseous Cl.sub.2 is absorbed into the BHP and reacts to form the highly energetic O.sub.2 (.sup.1 .DELTA.) which diffuses out of the BHP into the gaseous phase. The O.sub.2 (.sup.1 .DELTA.) has a strong tendency to be quenched by pooling the energy of two O.sub.2 (.sup.1 .DELTA.) molecules to form an even more energetic specie known as oxygen singlet sigma, (O.sub.2 (.sup.1 .SIGMA.)). The O.sub.2 (.sup.1 .SIGMA.) quickly returns to the O.sub.2 (.sup.1 .DELTA.) state, with the overall process resulting in a net loss of the equivalent stored energy of one O.sub.2 (.sup.1 .DELTA.) molecule. As a consequence of this pooling loss process, the reactors producing O.sub.2 (.sup.1 .DELTA.) must be operated at low O.sub.2 (.sup.1 .DELTA.) pressure and short gas residence time to avoid excessive losses. The need for short residence time encourages the designer to use fast gas velocity in the reactor, but carryover of liquid with the O.sub.2 (.sup.1 .DELTA.) product gas sets a limit on the maximum gas velocity which can be used. The need for low O.sub.2 (.sup.1 .DELTA.) pressure encourages the designer to employ low pressure reactors. However, a vacuum system is needed to exhaust all laser exhaust. The lower the reactor pressure, the lower the vacuum pressure must be, leading to a larger and more costly vacuum system. Typically, the Cl.sub.2 reactant is mixed with a diluent to raise the overall pressure of the reactor while keeping the O.sub.2 (.sup.1 .DELTA.) pressure low. To minimize gas density in the reactor, a low molecular weight diluent is selected, and it is most often helium.
Downstream of the reactor producing O.sub.2 (.sup.1 .DELTA.), iodine is mixed into the oxygen (see e.g., U.S. Pat. No. 4,653,062). Usually the iodine is in the form of diatomic molecules, (I.sub.2). The iodine is quickly dissociated into iodine atoms by a process involving O.sub.2 (.sup.1 .DELTA.). Iodine atoms have an elevated energy level very near to the difference in energy levels of O.sub.2 (.sup.1 .DELTA.) and ground state oxygen (O.sub.2.sup.3 .SIGMA.). The energy of O.sub.2 (.sup.1 .DELTA.) is quickly interchanged back and forth with the iodine atoms in a process which is near to equilibrium. The excited iodine atoms, often designated as I*, are the lasing specie used to extract energy from the mixed gases. The equilibrium fraction of iodine atoms in the I* state increases as the gas temperature is lowered. The I* specie is unfortunately quenched to the ground state very quickly, with conversion of the energy to heat, by several species, the specie of most concern being water vapor (from the BHP) which accompanies O.sub.2 (.sup.1 .DELTA.) effluent from the reactor. The I* quenching issue is most important in the region between the I.sub.2 injection location and the start of the laser cavity (the "dissociation zone"), Consequently, the length of the dissociation zone is kept as small as practical by the COIL designer to minimize the losses due to I* quenching. The issue is less pronounced in the laser cavity because the lasing process reduces fraction of I* and hence the rate of quenching is reduced.
Because the equilibrium toward I* is more pronounced at low temperature, there is a power extraction efficiency advantage to running the laser cavity at low temperature. This is typically achieved by accelerating the gases to supersonic Mach numbers. As higher Mach numbers are selected, the pressure in the cavity must be lowered (rather than increasing the gas source pressure), since the available O.sub.2 (.sup.1 .DELTA.) pressure is limited by the need to keep the reactor producing O.sub.2 (.sup.1 .DELTA.) at low pressure as discussed above. The major disadvantage of higher Mach numbers is the impact on the vacuum system needed to operate the laser. Generally after laser energy extraction the high Mach number gases are passed into a diffuser where pressure is recovered from the high Mach number momentum. The typical COIL operates at a supersonic cavity Mach number of about 2, which is a compromise between higher laser power extraction at low temperatures corresponding to higher Mach numbers, and less pressure recovery at higher Mach numbers, and lower cavity pressure which leads to less pressure recovery at higher Mach numbers. Oftentimes, the iodine injection/mixing/dissociation function and the nozzle for accelerating the gas to Mach 2 are combined in the same piece of hardware, since the dissociation zone is best kept small.
The three most difficult issues related to COIL devices are (1) the avoidance of liquid BHP carryover (which scatters laser light and produces deleterious water vapor) with the reactor effluent, (2) the low pressure recovery which results in a large exhaust vacuum system, and (3) achievement of high overall efficiency (defined in terms of the ratio of laser power output to Cl.sub.2 gas input).
Direct substitution of nitrogen diluent for the usual helium diluent in a COIL device reactor producing singlet delta oxygen generator results in a low flow capacity due to the effects of higher gas density leading to carryover of the liquid basic hydrogen peroxide into the laser cavity. As a consequence, it is necessary to significantly reduce the throughput of the laser device. Furthermore, the high gas density requires a lower gas flow velocity to avoid liquid carryover. The low velocity leads to decay of the singlet delta oxygen before it reaches the laser cavity. The net result is reduction by almost 50% in total flow capacity, and a further reduction of approximately 50% in laser efficiency. The same device, thus produces only about 1/4th the total laser power.